1. Field of the Invention
The present invention relates broadly to positive planographic printing materials having sensitivity in the infrared wavelength range, and particularly, to positive planographic printing materials which are capable of being used for what is known as "direct plate making" using an infrared laser based on digital signals outputted from a computer or the like.
2. Description of Related Art
There have been remarkable developments in lasers in recent years. In particular solid lasers or semiconductor lasers emitting infrared rays having wavelengths of 760 to 1200 nm (herein after sometimes referred to as "infrared laser"), and having a compact size and high output are now readily available. These infrared lasers are very useful as a recording light source when making a direct printing plate based on digital signals outputted from a computer or the like. Therefore, recent years have seen an increasing desire for an image recording material having high sensitivity to the above types of infrared ray recording light sources, namely, an image recording material in which a photochemical reaction and the like occurs upon irradiation with infrared rays and solubility in a developing solution varies significantly.
An image recording material of this type which can be recorded on by an infrared laser is the recording material constituted of onium salts, phenol resins and spectral sensitizers described in U.S. Pat. No. 4,708,925. This image recording material is a positive image recording material utilizing the effect manifested by a onium salt and a phenol resin of suppressing the dissolution of the image recording material in a developing solution.
It is difficult to cause photoreaction and the like in a compound wherein the solubility of an image recording material in a developing solution varies significantly by exposure with an infrared ray since the infrared ray has less energy than the ultraviolet ray conventionally used as a light source for exposure. For example, International Publication (WO) No. 97/39894 discloses an image recording material composed of an infrared absorber and a binder polymer soluble in an aqueous alkaline solution. In this image recording material, there is excellent positive action (development is suppressed in unexposed portions, but the development-suppression disappears in exposed portions) occurring as a result of light-to-heat conversion on the surface of the material irradiated with a laser, however, heat generated near the surface does not reach sufficiently deep into the material and positive action in deep portions is insufficient. As a result, even if alkaline development is conducted, discrimination between unexposed portions and exposed portions is not easily made clearly and sensitivity and development latitude are problematical.
Further, Japanese Patent Application Laid-open (JP-A) No. 7-20629 proposes an example in which a diazonium salt having a high level of heat decomposability is added as light/heat decomposable substance. This sensitive material has improved sensitivity and development latitude with regard to an infrared ray laser, however, it has poor storage stability such that the sensitivity decrease with time and is also problematical with regard to product management and the like.
As a result of intensive research into the constituent components of a positive planographic printing material capable of directly making a printing plate by the irradiation thereof with an infrared ray, the present inventors have found the following facts. When a polyfunctional amine compound is used together with a polymer which is water-insoluble but soluble in an aqueous alkaline solution (hereinafter, may be referred to simply as "aqueous alkali solution-soluble polymer"), the above-described polyfunctional amine compound causes a strong interaction with an alkali-soluble group in the above-described aqueous alkali solution-soluble polymer. Consequently, the film structure of a photosensitive film containing these compounds is fixed in a stable fashion. During image formation, non-image areas become alkali-soluble due to the release of the above-described interaction by the energy of an infrared ray and the like from the exterior, and in the present invention, it is thought that, because the interaction is a complex-complementary interaction caused by the aqueous alkali solution-soluble polymer, the infrared absorber, and the multi-functional amine compound, a greater release than when a normal combination of an aqueous alkali solution-soluble polymer and an infrared absorber is used occurs in non-image portions. Thus, the present inventors have found that high sensitivity and discrimination in image formation can be attained by the present invention. Further, it was found that changes in solubility with the passage of time and changes in the sensitivity of a planographic printing material can be suppressed, thus completing the present invention.